Embodiments described herein relate in general to the field of geological mapping, and more particularly to systems for conducting electromagnetic surveys.
Geophysical electromagnetic (“EM”) techniques can be effective in determining the electrical conductivity of soils, rocks and other conductive material at depths from the surface of up to about three kilometers. Conductivity distribution at depths is of great interest to those involved in mapping base metals and uranium deposits, aquifers and other geological formations.
Geophysical EM methods can involve measurements of time-varying secondary magnetic fields near the earth's surface. These secondary fields may be produced by way of a primary magnetic field. The source for the primary magnetic field may be current applied to a transmitter (for example in an active EM surveying system), or by naturally occurring electromagnetic fields originating mainly from lightning in the earth's atmosphere (for example in a passive EM surveying system). EM fields can have a characteristic ground penetration depth proportional to the inverse of the square-root of both ground conductivity and frequency.
Traditionally the secondary magnetic field signal can be measured using either a receiver coil system (which for example can be used to measure the magnetic field time derivative dB/dt), or a magnetometer (which measures the magnetic field B). The received analog signal may then be amplified, filtered, and digitized by a high-resolution high-speed analog-to-digital converter (ADC), and the data can be stored along with the positioning information obtained from a Global Positioning System (GPS). Data post-processing may involve electrical and physical modeling of the ground to generate the geophysical conductivity contour maps.
Geophysical surveys may typically require high signal-to-noise ratio (SNR), high conductivity discrimination, and high spatial resolution both laterally and in depth. The EM survey requirements may differ depending on ground conductivity and the desired probing depth.
Some EM systems may use either or both ground-based EM measurements and airborne measurements using airplanes and helicopters. Airborne methods may be preferred for large area surveys and can be used for exploration of conductive ore bodies buried in resistive bedrock, geological mapping, hydrogeology, and environmental monitoring.
For airborne electromagnetic (“AEM”) systems, the data may be acquired while the airplane or helicopter flies at a nearly constant speed (for example up to 75 m/s or 30 m/s, respectively) along nearly-parallel and close to equally-spaced lines (for example 5 m to 200 m) at an approximately constant height above ground (for example about 120 m or 30 m, respectively). Measurements can be taken at regular intervals, for example in the range of 1 m up to 100 m.
In active systems having a transmitter coil, EM measurements can be recorded either in the frequency domain or time domain. In frequency-domain electromagnetic (“FDEM”) measurements, the transmitter coil continuously may transmit an electromagnetic signal at fixed multiple frequencies, while the receiver coil measures the signal continuously over time. The measured quantities may for example be either signal amplitude and phase as a function of frequency, or equivalently, the in-phase and in-quadrature amplitudes as a function of frequency.
The measured quantities will be affected by the conductivity and geometry of geological bodies in the ground, and can be processed to yield conductivity contour maps. In time-domain electromagnetic (“TDEM”) measurements, a pulse of current may be applied to be transmitter coil during an on-period and switched off during an off-period, typically at a repetition rate equal to an odd multiple of half of the local power line frequency. The signal may be measured at the receiver as a function of time. The small amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, can be utilized to yield the conductivity contour maps.
In passive systems that rely on Audio Frequency Magnetic (“AFMAG”) measurements, naturally occurring EM fields produced by global lightning discharges maybe used as the excitation source. These EM fields propagate around the earth as plane waves guided by the ionosphere and earth's surface. In AFMAG, the resulting EM signal may be measured by the airborne receiver coil, for example in the frequency range 25 Hz to 2 kHz with data acquisition at 6.25 kHz with 24 bits resolution. In some AFMAG survey systems the measured signal may then be separated into frequency bands such that the crossover frequencies between each band and the next higher band are in the approximate ratio of 1.5:1, and then processed to produce conductivity contour maps. AFMAG may also use measurements of the horizontal magnetic field in real time in order to normalize the measurements done in the aerial survey as the source intensity is constantly varying.
One possible AFMAG setup is to use two orthogonal coils at the ground base station to yield the horizontal component of the magnetic field, and one flying coil to measure the vertical component of the magnetic field. A second vertical coil can also be used at the ground base station to improve the measurement of the reference signal. In some systems, the aerial measurements can be made typically at a distance less than 50 km from the ground base station.
Alternative setups can use three-component measurements at the ground station and/or three component measurements obtained in flight. Multiple base stations can also be used to locate the EM field source and improve the SNR of the measurements.
A receiver coil may measure dB/dt directly. The voltage induced in the receiver coil by a magnetic field B is given by N.A. dB/dt, where the coil sensitivity N.A is the product of the coil number of turns N and the coil area A, and dB/dt is the time-derivative of the magnetic field. The inductance of a coil is proportional to N2D, where N is the number of turns and D is the effective diameter of the coil.
Sources of electrical noise at the receiver coil can include, among other things, the spurious signals produced by the towing aircraft (such as a helicopter) and other metallic parts of the system, lightning activity in the atmosphere, local AC power line interference, VLF radio waves, and thermal noise from the coil and the electronics. A prominent source of noise at the airborne revolver coil may be the microphonic noise produced by the motion of the coil in the magnetic field of the earth. The motion can be produced by wind buffeting the coil, vibration from the aircraft, and rubbing of the coil against the coil suspension system.
U.S. Pat. No. 6,876,202 entitled “System, Method and Computer Product Geological Surveying Utilizing Natural Electromagnetic Fields”, issued to Edward Beverly Morrison and Petr Valentinovich Kuzmin, granted 2005 Apr. 5 discloses a receiver coil and suspension means that facilitates a reduction of microphonic noise produced by mechanical vibrations of the receiver coil in the magnetic field of the earth. The method applied by U.S. Pat. No. 6,876,202 is to surround the coil with an acoustic noise absorber. It also discloses a means of reducing noise through permitting distance between the sensors and the aircraft.
U.S. Pat. No. 7,157,914, which includes a description of a geohysical survey system that includes transmitter and receiver coils, discloses a double-suspension receiver suspension apparatus for reducing vibration and microphonic noise.
Increasing signal-to-noise ratio (SNR) at the receiver coil may not be straightforward due to many factors affecting the measurement. In order to minimize the noise produced by various sources in the frequency range of interest, one may need to reduce the movement of the receiver coil relative to the magnetic field of the earth, prevent external mechanical noises from reaching the receiver coil, and minimize the mechanical noises produced by the receiver coil suspension system.
Thus a double-suspension receiver coil that further ameliorates noise in a receiver coil is desired.